(a) Field of the Invention
The present invention relates to a blind rate detection method and device in an asynchronous mobile communication system. More specifically, the present invention relates to a blind rate detection method and device in an asynchronous mobile communication system with simpler hardwired implementation and improved reliability.
(b) Description of the Related Art
In general, a transmitter in an asynchronous mobile communication system performs a channel encoding process and a modulation process on blind rate data to transmit them to a receiver via a radio channel.
The receiver receives the blind rate data from the transmitter through the radio channel, a demodulator demodulates them, and a Viterbi decoder performs a decoding process on them.
In this instance, a transmission speed of input data must be previously known in order for the Viterbi decoder to decode symbols of the received data.
Transmission speed information of image data or control signals but not speech data is transmitted to the decoder through various paths, for example, an additional channel or a transport format combination indicator (TFCI).
However, in the case of the speech data, blind rate information is not previously transmitted to the decoder. Therefore, in order to determine various speeds transmitted by the transmitter, the Viterbi decoder uses information such as a cyclic redundancy code (CRC) check to determine a transmission speed. Hence, in the case there is no TFCI, the asynchronous IMT-2000 system requires an algorithm for detecting a blind rate, that is, a blind transport format detection (BTFD) algorithm.
A conventional BTFD algorithm for detecting the blind rate is represented in FIG. 1, but prior to describing the BTFD algorithm, a data structure that has a blind rate will be described with reference to FIG. 2.
Referring to FIG. 2, the variable length data stream undergoes block encoding and convolutional encoding using a CRC, and CRC parity bits are provided at the end of the variable length data stream in a reverse order. The receiver is notified of a transmittable format or a last bit's possible position Nend, which can be plural, and it is now restricted to nine positions, each of which is defined to be 39, 42, 49, 55, 58, 61, 65, 75, and 81.
First, various parameters needed for detecting the blind rate are reset in step S101. Here, the parameters include a last bit's possible position Nend, a minimum value Smin of path selection values, and a last detection position Ndetected—end. The last bit's possible position Nend and the last detection position Ndetected—end are reset to be 1, and the minimum value Smin of the path selection values is reset to be a path selection threshold value THpath-selection that is previously set as a predetermined value.
Next, Viterbi decoding is performed, and an accurate trellis path is terminated in the 0 state at the last bit's possible position Nend in step S103. In this step, the Viterbi decoder performs add, compare, and select operations (i.e., ACS) to generate a maximum path metric value amax, a minimum path metric value amin at the last bit's possible position Nend, and a path metric value a0 in the 0 state.
Next, a path selection value S(Nend) at the last bit's possible position Nend is found by Equation 1 in step S105.
                              S          ⁡                      (                          N              end                        )                          =                  10          ⁢                      log            ⁡                          (                                                                                          a                      max                                        ⁡                                          (                                              N                        end                                            )                                                        -                                                            a                      0                                        ⁡                                          (                                              N                        end                                            )                                                                                                                                  a                      max                                        ⁡                                          (                                              N                        end                                            )                                                        -                                                            a                      min                                        ⁡                                          (                                              N                        end                                            )                                                                                  )                                                          Equation        ⁢                                  ⁢        1            
Next, in order to remove the generation of a detection error, it is determined whether the path selection value S(Nend) at the last bit's possible position Nend is equal to or less than the path selection threshold value THpath-selection in step S107. That is, according to the path selection threshold value THpath-selection, it is determined whether the trellis path connected to the 0 state is to be traced back to the last bit's possible position Nend.
If the path selection value S(Nend) fails to satisfy Equation 1, that is, when the path selection value S(Nend) is greater than the path selection threshold value THpath-selection, it is determined whether the last bit's possible position Nend is a maximum value in step S119. When the last bit's possible position Nend is not the maximum value, the last bit's possible position Nend is increased by 1 in step S121, and the steps S103, S105, S107, and S119 are repeated so that the last bit's possible position Nend may be the maximum value, that is, 81. Accordingly, when the last bit's possible position Nend is the maximum value, the maximum value is output to the last detection position Ndetected—end in step S123.
When the path selection value S(Nend) satisfies Equation 1, that is, when the path selection value S(Nend) is less than or equal to the path selection threshold value THpath-selection, its path is traced back from the last bit's possible position Nend so as to restore frame data in step S109.
Next, the CRC parity is calculated from the restored data in step S111, and a CRC check is performed in step S113.
If a CRC error occurs in the CRC check step S113, the step S119 is executed, and if no CRC error occurs, it is determined in step S115 whether the path selection value S(Nend) at the last bit's possible position Nend is less than the minimum value Smin of the path selection values reset in the reset step S101.
In this instance, when the path selection value S(Nend) is equal to or greater than the minimum value Smin of the path selection values, the step S119 is executed, and if the path selection value S(Nend) is less than the minimum value Smin of the path selection values, the path selection value S(Nend) of the last bit's possible position Nend is stored as the minimum value Smin of the path selection values, the last bit's possible position Nend is stored as the last detection position Ndetected—end in step S117, and the step S119 is executed.
As described, the conventional blind rate detection algorithm uses a logarithmic function as shown in the previous step S105, and since it is very complicated to implement the logarithmic function in hardware, the hardware requires an unnecessarily large amount of storage.
Further, when the CRC check is passed in the previous step S113, the path selection value S(Nend) of the last bit's possible position Nend is routinely compared to the minimum value Smin of the path selection values, and hence, the last detection position Ndetected—end cannot be accurately found under a very bad channel environment.